Ornamental Koi are mostly bred in Japan and shipped to rest of the world. Many years of selective breeding had led to development of various colourful varieties of Koi but all will need the same water requirements. They are relatively hardy fish and can tolerate poor water conditions but not for prolonged periods. If you are serious about Koi keeping, then continual good water quality is must to maintain your Living Jewels at their peak condition.
Size
Koi can grow to as large as 1 metre in length, although most Koi sold are young Koi and averages from 20 cm (yearlings) to about 50 cm. Koi need space to thrive and some Koi experts have claimed that growth rate is proportional to pond size and volume of water per Koi. Just like in aquarium fish keeping, a larger volume of water or pond would also provides more stability in the water condition. Often, this is limited by available space for the pond and not by cost. I am not sure about other places, but in Singapore, my pond builder has told me that I can have a slightly larger (or smaller) pond at the same price (like buying a shirt!). If you are planning to keep 9 Koi (each about 70 cm long), a decent-sized pond would be approximately that of a king-size bed. Believe me, once you are bitten by the Koi-keeping bug, you would not want to stop at 9!
Oxygen level
Koi needs oxygen and so does the bacteria in the biological filter. Dissolved Oxygen level of 8 mg/litre in water temperature of about 25 deg C is almost ideal since it is almost at saturation point. I used a TetraPond water test kit to measure this. Note also the plants (including algae) in the water also uses oxygen at night. Plants take in carbon dioxide and produce oxygen only in the day, during photosynthesis.
Temperature
There is a need to keep the water cool in hot, sunny Singapore where the daily temperature ranges from 25 deg C at night to 34 deg C in the day. Warmer water does not carry oxygen as well as cooler water. In any case, some shade would be good for the Koi. I have read about Koi getting sun-burn from prolong exposure to direct sunlight.
Pond Depth
Pond depth should ideally be at least 1.2 meter or about 4 feet. A deeper pond is good for the Koi growth but one has to be mindful of the proportion of the depth versus size of the pond. You find the Divine Ratio an interesting read (http://goldennumber.net/architecture.htm) but this is not a must-have pond building requirement! Also, a deeper pond would allow for greater pond volume per unit pond surface area. Pond surface area is usually limited by the space available in our home or garden. A deeper pond would also help keeps the water cooler in hot, sunny weather.
pH level
pH is a measure of the acidic or basic (alkaline) nature of a solution. Normal tap water or fresh water should have PH level of 7. Water with a lower PH level is deemed acidic while alkaline water would have higher pH. Chalk, limestone, coral or seashells dissolved in water will produce higher pH. My experience will aquarium fish has taught me that importance of maintaining the right pH level for the right type of fish. For example, African cichlids which originates from Lake Malawi requires high pH and a rocky environment, while South American tetras and cichlids which originates from the rivers of Amazon requires low pH. Low pH or sudden drop in pH level kills Koi and many other fishes. It is generally better to have pond water for Koi at with slightly higher pH (7.4 is ideal. Not more than 8.0).
Ammonia, Nitrite and Nitrate
Fish produces ammonia as a waste product and a good filter system must be provided to remove this ammonia from the water. If a biological filter is used, the bacterial in the filter will convert the ammonia to nitrite and then from nitrite to nitrate. Nitrate is relatively less harmful to fish and Koi than ammonia or nitrite. Ideal levels of ammonia and nitrite should be zero or near zero and this can be achieved using a good biological filter system. Nitrate level should be less than 25 mg/litre. The only way to reduce nitrate is through partial water changes and also use of water plants. Nitrate is commonly found in garden fertilisers (the N in the NPK fertiliser).
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